The present invention relates to a semiconductor laser used as a light source for an optical disk device such as a CD-ROM, a CD-R, or a DVD-ROM, a high-definition laser beam printer (LBP), a laser pointer, or the like. More specifically, it relates to such a semiconductor laser that can oscillate at a high power without generating a kink by constricting a current by use of the current constriction layer as well as confining the light as much as possible, or that can oscillate at a high power without generating a kink while being self-excited at a low power by confining the light as well to some extent.
A self-alignment type semiconductor laser provided with a light confinement effect due to a current constriction layer has such a construction as shown in FIG. 4 as an example. That is, as shown in FIG. 4, on a substrate 21 made of, e.g., an n-type GaAs are sequentially grown epitaxially an n-type clad layer 22 made of, e.g., n-type Al0.6Ga0.4As, an active layer 23 made of non-doped Al0.2Ga0.8As, a p-type first clad layer 24a made of p-type Al0.6Ga0.4As, an etching stopping layer 25, a current constriction layer 26 made of, e.g., n-type Al0.7Ga0.3As, a p-type second clad layer 24b made of p-type Al0.6Ga0.4As, and a p-type contact layer 27 made of GaAs, on the top surface of which is formed a p side electrode 28 and on the back surface of the GaAs substrate 21 is formed an n side electrode 29, so that the resultant wafer is subdivided into chips by cleavage or the like to thereby form a semiconductor laser (LD) chip having a construction shown in FIG. 4.
This construction employs, to use the laser for write-in operations or the like, a method for disposing the current constriction layer near the active layer or enlarge the mixed-crystal ratio of Al in the current constriction layer to provide an effective difference in refractive index in order to enhance the light confinement effect, thus oscillating a laser at a high power.
Also, to use the laser for read-out operations or the like, such a method is required to be employed that the noise is reduced at a low power and, for self excitement, the mixed-crystal ratio of Al in the current constriction layer is reduced or the current constriction layer is disposed distant from the active layer to thereby relax the light confinement effect in order to spread the light, thus enabling forming a supersaturating absorption layer outside a current implanting region in the active layer.
As mentioned above, in order to enhance the light confinement effects and oscillate a laser at a high power, the current constriction layer must be disposed near the active layer as much as possible or the mixed-crystal ratio of Al in the current constriction layer made of an AlGaAs-based compound semiconductor must be enlarged to thereby reduce the refractive index. If the mixed-crystal ratio of Al is enlarged, however, the exposed surface of the current constriction layer after a stripe trench is formed therein easily corrodes because Al is very easily oxidized, thus suffering from a problem that a clean mono-crystal semiconductor layer cannot easily be grown when a semiconductor layer is grown again. Although by, in particular, forming beforehand a protective layer such as made of GaAs on the top-most surface of the current constriction layer, thermal etching can be carried out before the re-growing, thus providing a clean layer, the side walls of the stripe trench cannot be cleaned in such a way, so that the semiconductor layer is liable to be poly-crystallized to thereby flow a leakage current and so increase the threshold current value, thus leading to a problem of an increased electric resistance due to poly-crystallization and also a rise in the operation current.
Since the current constriction layer, on the other hand, has a conductivity type different from that of its surrounding clad layer to thereby prevent a current flow by the reverse-biased pn junction, the pn-junction portion has a depletion layer formed thereon due to the reverse biasing, so that as shown in FIG. 5, if the current constriction layer 26 is formed too close to the active layer 23, the depletion layer (refer to C in FIG. 5) reaches the active layer 23. If the depletion layer C reaches to the active layer 23, as shown in FIG. 5, a current I flows to the portion of the current constriction layer 26 where no stripe trench is formed to disable from constricting the current, thus leading to a problem that the invalid current flows through the active layer.
Although to use a laser for both write-in and read-out operations, on the other hand, the laser must oscillate at a high power without generating a kink while being self-excited at a low power to thereby reduce the noise, as mentioned above, there is a trade-off relationship between self-excitement at a low power and obtaining a high power and, therefore, both requirements cannot be satisfied at the same time, so that a self-excitement type semiconductor laser, which has a large fluctuation in power, suffers from a phenomenon called a kink that the power drops during the process of increasing the operation current, leading to a problem of difficulty in obtaining of a high power. A semiconductor laser for obtaining a high power, on the other hand, cannot be self-excited, leading to a problem that the noise cannot be suppressed.
In view of the above, it is an object of the invention to provide a self-excitement type, high-power semiconductor laser that can operate in a stable manner even at a high power without generating a kink while being self-excited at a low power.
It is another object of the invention to provide a high-power semiconductor laser which can operates in a stable manner even at a high power with no kink generated without enlarging so much the mixed-crystal ratio of Al in the current constriction layer and also with preventing a depletion layer due to a pn junction of the current constriction layer from reaching the active layer.
A semiconductor laser according to the invention comprises an active layer sandwiched by n-type and p-type clad layers in such a construction that either one of the above-mentioned clad layers is provided with a current constriction layer consisting of at least two layers for current confinement and light confinement, the first layer of the current constriction layer closer to the above-mentioned active layer has a conductivity type different from that of the clad layer provided with the current constriction layer and being made of a material having almost the same refractive index as that of the clad layer and the second layer of the above-mentioned current constriction layer farther from the active layer being made of a material having a refractive index smaller than that of the first layer.
By this construction, the first layer closer to the active layer has almost the same refractive index as that of the clad layer to thereby eliminate the light confinement effect, thus serving as a layer only for constricting the current. The second layer farther from the active layer, on the other hand, has a smaller refractive index than the first layer and has the light confinement effect. In this case, the light is emitted from the current implanting region constricted by the first layer closer to the active layer and then the second layer confines the light in the light emitting region from which the light was oscillated, so that the light emitting region in which the light is confined becomes larger than the region to which a carrier is implanted to emit the light thereby permitting thus enlarged portion of the active layer to act as an supersaturating absorption layer. Accordingly, the light can be confined in an enhanced manner while reserving that supersaturating absorption layer, to sufficiently confine the light during self-excitement by use of the supersaturating absorption layer, thus obtaining a high power without generating a kink. Note here that the second layer does not have to be different in conductivity type from the surrounding clad layer but may have the same conductivity type if the first layer can confine the current sufficiently.
In the event that, for example, AlGaAs based compound semiconductor for emitting the infrared light or InGaAlP based compound semiconductor for emitting the red light forms a double hetero construction in which the active layer is sandwiched by n-type and p-type clad layers having a larger band gap than the active layer, the first layer of the current constriction layer is made of a material having almost the same composition as that of the clad layer of the AlGaAs based or InGaALP based compound semiconductor, and the second layer is made of a material having an enlarged mixed-crystal ratio of Al to thereby provide an enlarged difference in refractive index, so that the larger the mixed-crystal ratio of Al or the closer to the active layer, the more the layer has the light confinement effect.
It is preferably that the first and second layers are so formed as to function mainly as a current confinement layer and a light confinement layer respectively and also the stripe trench provided to the first layer is so formed as to be smaller in width than that provided to the second layer, because the supersaturating absorption layer can be reserved securely. That is, if the stripe trench is formed not perpendicular to the surface of the semiconductor layer but is formed to have an inclined surface with respect to that, the stripe width of the first layer closer to the active layer is made smaller than that of the second layer. In this case, however, the two stripe trenches may be etched in different patterns to form the first layer narrower than the second layer.
Even if the stripe trenches are formed so as to have an inclined surface with respect to the width direction of the current constriction layer and the inclined surface of the first layer is formed so as to have a smaller inclination angle than that of the second layer, the stripe trench of the first layer has an even smaller width than the second layer, so that the supersaturating absorption layer can be expanded. Here, the inclination angle of the inclined surface refers to an angle xcex8 of the side wall of the stripe trench with respect to the growing surface of the semiconductor layer (see FIG. 1(b)).
The semiconductor laser according to the invention may have another construction that has a double hetero structure in which the active layer is sandwiched by n-type and p-type clad layers and either one of these clad layers is provided with a current constriction layer with a stripe trench having a different conductivity type from that of this clad layer and also at the current constriction layer on the side of the active layer a light confinement layer having s smaller refractive index than this clad layer is formed without doping or is formed so as to have the same conductivity type as this clad layer.
By this construction, the light confinement layer having a smaller refractive index is formed without doping or so as to have the same conductivity type as the clad layer, so that the only the light confinement effect can be obtained without forming a reverse-biased pn junction. Accordingly, the light confinement layer can be disposed close to the active layer unlimitedly. On the side of that light confinement layer opposite to the active layer, on the other hand, a current constriction layer having a different conductivity type from that of the clad layer is provided to thereby inhibit a current by the reverse-biased pn junction. A depletion region due to this reverse biasing would spread to the light confinement layer but can be controlled not to reach the active layer by adjusting the thickness of the light confinement layer, thus preventing a leakage current from flowing therethrough. As a result, it is possible to enhance the light confinement effect while sufficiently preventing the leakage current from flowing, thus providing stable operations up to a high power.
If the current constriction layer is formed of a semiconductor layer having the same refractive index as the light confinement layer, it is possible for this light confinement layer, even if it is thin, to prevent a depletion layer due to the reverse biasing from reaching the active layer while sufficiently confining the light together with the current constriction layer.